Astronauts from the space shuttle Endeavour recently attached the Alpha Magnetic Spectrometer, or AMS, to the International Space Station. It will attempt to detect the presence of antimatter in outer space. Since the device has the potential to change the way we think about the universe, this is a good time to brush up on what, exactly, antimatter is.
In the 1920s, British physicist Paul Dirac was trying to make Einstein’s special relativity principle jibe with some of the rules of quantum mechanics — a mathematical system that explains the behavior of small particles. No matter how many times Dirac ran his equations, he couldn’t eliminate a pesky negative sign that he thought didn’t belong there. Dirac ultimately decided the negative sign wasn’t a mistake, but a revelation. For the calculations to work, there had to be an undetected particle with the same weight as a negatively-charged electron — one of the basic building blocks of matter — but with a positive charge. Dirac thus became the first physicist to prove, albeit theoretically, that antimatter existed.